This invention relates generally to radio frequency energy jamming systems and more particularly to jamming systems which include a receiver to determine the frequency of a signal to be jammed.
As is known in the art, a so-called "spot-jammer" generally includes a radar receiver to determine the frequency of a radar signal to be jammed so that a jamming signal made up of a continuous wave (cw) carrier frequency centered at the frequency of the received radar signal and frequency modulated by a band-width limited noise waveform may be directed toward the source of the radar signal. This type of modulation is used because it generally makes most efficient use of the available power amplifier capabilities.
Since the transmitted jamming signal is generally much stronger than the received signal, the frequency of which must be determined to establish the center carrier frequency of the jamming signal, some provision is generally necessary to suppress or "look through" the jamming signal. One obvious way to do this is to interrupt the jamming signal periodically, and during the periods of interruption determine the frequency of the received radar signal. This technique, however, would degrade, significantly, the jamming effectiveness. It would be more desirable to be able to "look through" the jamming signal without interruption to the jamming signal transmission.
One technique used to "look through" the jamming signal includes separate antennas, one for the jamming transmitter and one for the radar signal receiver. The antennas are arranged so that the level of the jamming signal arriving at the receiving antenna is minimized. Such system includes a dual-channel superheterodyne receiver having a pair of mixers which use a portion of the jamming output as their local oscillator signal. Each one of the pair of mixers has an output coupled to a narrow band filter tuned to a suitable intermediate frequency. The outputs of the narrow band filters are fed to an automatic frequency control circuit (AFC) which produces a control signal to change the center carrier frequency of the jamming signal. After the system is locked to the frequency of the received signal by sweeping the control signal one of such filters produces a control signal when the jamming signal frequency is sufficiently higher than the frequency of the received signal, and the other one of such filters produces a control signal when the received signal frequency is sufficiently higher than the frequency of the jamming signal to track the frequency of the received signal. A source of band-limited noise is used to frequency modulate the carrier frequency of the jammer. Since isolation between the jamming transmitter antenna and the receiving antenna is not perfect, there will occur some leakage of the jamming signal into the receiver. This leakage signal will degrade the sensitivity of the receiver. That is, if at any small increment in time the frequency of the portion of the jamming signal were sufficiently different from that of the leakage jamming signal also fed to such mixers, a difference or beat frequency signal would be produced which would pass through one of the pair of filters and thereby cause the AFC to produce a control signal which would incorrectly change the center carrier frequency of the jamming signal. In order to provide some degree of compensation from leakage between the transmitter antenna and the receiving antenna, the portion of the jamming signal coupled to the pair of mixers passes through a delay line. This delay line is selected to match the delay of the leakage jamming signal and thereby cancel its effect on producing an erroneous control signal.
While the use of such a delay line provides some degree of cancellation of the leakage jamming signal, complete cancellation is not achieved because the external leakage path is diffuse. Consequently, receiver sensitivity is still degraded by any residual, uncancelled, leakage jamming signal.